This invention relates to microwave diode mixers for use at frequencies approximately within the range of 2 to 18 GHz and more particularly to symmetrical balanced two-diode mixers.
The operation which consists in mixing two sine-wave signals having different frequencies by means of one or a number of diodes is well-known.
In the type of mixer which is constructed with a single diode, the signal applied between the ends of the diode is the sum of an incident signal S having an angular frequency .omega..sub.S and of a signal having an angular frequency .omega..sub.L transmitted by a local oscillator. The resultant output signal has a spectrum composed of a plurality of angular frequencies .omega..sub.i.sbsb.mn =.+-.m.omega..sub.s .+-.n.omega..sub.L, where m and n are integers, each component of which can be isolated by a filter in order to be utilized.
Mixers having a number of diodes are usually of the symmetrical type, which means that only one-half of the diodes are conductive at any given moment. In the same manner as one-diode mixers, symmetrical mixers make it possible to obtain an intermediate frequency whose angular frequency represents the difference in angular frequencies of the fundamental components of the incident signal and of the signal formed by the local oscillator. Compared with one-diode mixers, symmetrical mixers have the advantages of suppressing certain intermodulation products in which the even harmonics of the signal appear, and of permitting recovery of the image frequency of the incident signal on condition, however, that they are perfectly balanced or in other words that the access signal or the signal of the local oscillator can be equally or symmetrically distributed in each diode.
In accordance with one method which may be mentioned by way of example, this state of balance is obtained in a symmetrical two-diode mixer by placing the diodes of the mixer at the terminals of the secondary winding of a midpoint or center-tap transformer. The incident signal is applied to the terminals of the primary winding and the signal delivered by the local oscillator is applied between the connecting point which is common to the two diodes and the midpoint of the transformer. A further advantage offered by this design is that, should the need arise, isolation could be provided between the ground line which carries the incident signal and the ground line which carries the signal of the local oscillator.
Unfortunately, a device of this type is not directly applicable to the construction of microwave mixers on account of the difficulties involved in the construction of wound transformers which are utilizable within these frequency ranges. The adaptation of this design solution to the microwave field calls for the use of a microwave circuit which, from a functional standpoint, is equivalent to the midpoint transformer employed for lower frequency ranges. In point of fact, microwave couplers having this property are already known and are designated as balun couplers, which is an abbreviation of the term balanced-unbalanced couplers. These devices are constructed either by means of coaxial lines or by means of waveguides constituted by microstrip circuits deposited by metallization on a substrate.
In the first form of construction, the coupler consists of a first coaxial line having a length equal to one-quarter of the wavelength of the incident signal and connected at one end to the inputs of two other coaxial lines, said inputs being coupled in parallel. In this configuration, the incident signal is applied to the free end of the quarter-wave coaxial line and the symmetrical signals are collected at the respective outputs of the other two coaxial lines.
In the other form of construction, the coupler is placed within a connection box and is constituted by a microstrip waveguide constituted by two parallel lines having a length equal to one-quarter of the wavelength of the incident signal. One line is connected to ground at the waveguide end to which the incident signal is applied. The symmetrical signals are collected at the other end of the waveguide respectively between each end of the lines and the ground of the circuit.
So far as dimensional considerations are concerned, the form of construction involving the use of a microstrip waveguide is often preferred to the design consisting of coaxial lines since it permits much smaller dimensions of the connection box. It is in fact sought to obtain maximum reduction of the dimensions of the mixer box in order to guard against parasitic resonance phenomena which may arise as soon as operation of the mixer takes place within a frequency zone in the vicinity of 10 GHz.
However, such a reduction no longer appears to be possible below a certain limit which, as explained earlier, appears to be closely related to one-quarter of the wavelength of the signal defining the length of the coupler and below which the signals collected at the output of the coupler are no longer symmetrical.
A further problem which claims attention and which interferes considerably with the reduction in dimensions of the mixer arises from the fact that balun-type type microstrip couplers do not permit the possibility of isolating the grounds of the local oscillator and of the incident signal, with the result that there is a potential danger of short-circuiting of the ends of the diodes connected to the output of the coupler or of grounding of the incident signal via the ground circuit which forms part of the local oscillator. These short-circuits can be prevented if a sufficient length of metallization separates the connection points of the diodes from the ground points on which the incident signal and the signal of the local oscillator are applied. This arrangement naturally prevents any reductions in dimensions of the desired mixer and increases the risks of parasitic resonance within the connection box.